Preheat your oven to 350 F degrees and line a cupcake pan with liners.
In a medium bowl, mash the large over-ripe banana until smooth. Whisk in the whole milk, vegetable oil, vanilla, egg, and sugar.
In a separate large bowl, whisk together the all-purpose flour, dark chocolate cocoa powder, baking soda, baking powder, salt, and cinnamon.
Make a well in the center of the dry ingredients and pour in the wet mixture. Gently whisk together until just combined.
Fill each cupcake liner about two-thirds full with batter.
Place the cupcake pan in the preheated oven and bake for 17 to 19 minutes. A toothpick inserted should come out clean.
Allow the cupcakes to cool in the pan for 2 to 3 minutes, then transfer to a wire rack to cool completely.
For the frosting, cream the unsalted butter until smooth. Add the mashed over-ripe banana, vanilla, and salt.
Add one cup of the powdered sugar and mix on low until incorporated. Gradually add the remaining powdered sugar and dark chocolate cocoa powder.
Beat on high speed for 1 to 2 minutes until fluffy and light in color.
Fill a piping bag with the frosting and pipe onto each cooled cupcake, garnishing with a slice of banana.